Originally Posted by Jetboy

I think the best advice is above: Don't overheat it. I'm not actually sure what it would take because I could tow my 5500lb travel trailer over the biggest mountain passes in the southwest at 100* summer temps and the engine temp never moved more than plus or minus 1-3 degrees - but if you can find some way to overheat a 4Runner engine - don't do it. For perspective on the radiator - the 4Runner radiator has about 580 square inches of radiator cooling area. An F150 with the 3.5EB has about 510 square inches. They are very comparable in size. But the 3.5 EB will be generating a lot more heat. Probably twice as much. The 4R cooling system is very large relative to the cooling demand. A Tundra 5.7 radiator is around 700. A Powerstroke F250 is about 810. Toyota builds to a higher standard of reserve capacity.

I don't know that acidity is a big issue with modern extended life coolants. High quality carboxylate-based coolants like basically all new OEM coolants last a long time. They do not use silicate based protection and so they do not need to be refreshed with new fluid with a fresh level of silicate rust inhibitor additive every few years like legacy green silicate based antifreeze did in the past. Do they last forever? no. But 100k miles should be no problem. Especially in an engine like a 1GR that isn't going to have extreme heat. Few people ever flush or change coolant. And very few engines have problems resulting from that.